85 Celica Fuel Pressure Problem

[Denis]

I am looking for some ideas about a fuel pressure problem I have with my 1985 Celica Convertible. The fuel pressure decreases from normal operating pressure to less than 5psi during approximately 1-2 minutes and after several minutes of vehicle running. I have followed shop manual tests including "Check fuel pump operation" and "Check fuel pressure"; shorted the fuel pump check connector and verified there is 33-38 psi (normal) fuel pressure requirement. At idle when short is removed the pressure operates between 27psi and 31psi (normal). These tests were conducted for at least 20 minutes continuous. This confirms the fuel pump (new), fuel filter (new), and pressure regulator (new) operates correctly. The short also bypasses the air flow meter and the ecu. On chance, I replaced the air flow meter and ecu with spare components. There was no change in the symptoms. This did not correct the problem. A final note, when fuel pressure drops during operation, I have confirmed the voltage at the fuel pump remains between 12 and 13 volts. Sure could use some help here with this one ... Thanks